Issue |
A&A
Volume 449, Number 2, April II 2006
|
|
---|---|---|
Page(s) | 711 - 722 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20054298 | |
Published online | 21 March 2006 |
Reduced Wolf-Rayet line luminosities at low metallicity
Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Sheffield, S3 7RH, UK e-mail: Paul.Crowther@sheffield.ac.uk
Received:
3
October
2005
Accepted:
5
December
2005
New NTT/EMMI spectrophotometry of single WN2–5 stars in
the Small and Large Magellanic Clouds are presented, from which He ii λ4686 line luminosities have been derived, and compared
with observations of other Magellanic Cloud Wolf-Rayet stars.
SMC WN3–4 stars possess line luminosities which are a factor of 4 times lower than LMC counterparts, incorporating
several binary SMC WN3–4 stars from the literature. Similar results
are found for WN5–6 stars, despite reduced statistics,
incorporating observations of single LMC WN5–9 stars
from the literature. C iv λ5808 line luminosities of
carbon sequence WR stars in the SMC and IC 1613 (both WO subtypes)
from the recent literature are a factor of 3 lower than LMC WC stars from
Mt Stromlo/DBS spectrophotometry, although similar
results are also obtained for the sole LMC WO star. We demonstrate how
reduced line luminosities at low metallicity follow naturally
if WR winds are metallicity-dependent, as recent empirical and
theoretical results suggest. We apply mass loss-metallicity
scalings to atmospheric non-LTE models of Milky Way and LMC WR stars
to predict the wind signatures of WR stars in the metal-poor star forming WR galaxy
I Zw 18. WN He ii λ4686 line luminosities are
7–20 times lower than in metal-rich counterparts of identical bolometric luminosity, whilst
WC C iv line luminosities are 3–6 times lower.
Significant He+ Lyman continuum fluxes are predicted for metal-poor early-type WR stars. Consequently, our results suggest a
larger
population of WR stars in I Zw 18 than is presently assumed, particularly
for WN stars, potentially posing a severe challenge to evolutionary models at very low metallicity. Finally, reduced wind strengths from WR stars at low metallicities impacts upon the immediate
circumstellar environment of long duration GRB afterglows, particularly
since the host galaxies of high-redshift GRBs tend to be metal-poor.
Key words: stars: Wolf-Rayet / galaxies: stellar content / galaxies: individual: I Zw 18 / stars: atmospheres
© ESO, 2006
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